Because of the high package density attainable with multilayer ceramic (MLC) substrate circuit structure, it has achieved extensive acceptance in the electronics industry for the packaging of integrated circuit semiconductor devices and other elements. In general, such conventional ceramic structures are formed from ceramic green sheets which are prepared from ceramic slurry. The slurry is made by mixing a ceramic particulate, a thermoplastic polymer (e.g. polyvinylbutyral) and solvents for the polymer. This slurry is then cast or doctor bladed into ceramic sheets from which the solvents are subsequently volatilized to provide a coherent and self-supporting flexible green sheet, which may be finally fired to drive off the binder resin and sinter the ceramic particulates together into a densified ceramic unitary substrate.
In the fabrication of multilevel ceramic structure, the green sheets are first punched to form via holes. Subsequently, a pattern of conductive material is deposited in the via holes and on the surfaces of the sheets. The green sheets are then assembled in the proper order and laminated wherein the metallurgy in the via holes and on the green sheets collectively form a complex internal metallurgy network. After the composite substrates have been pressed to adhere the sheets firmly together and indent the conductive metal patterns into the opposed sheets, the substrate is fired in an appropriate atmosphere at a temperature to first burn off the organic binder, and subsequently to sinter the particles of the substrate together to form a unitary ceramic substrate. The original green ceramic sheets are formed on a larger scale so that upon shrinking the spacing of the various elements conforms to the desired standards. The fabrication of multilayer ceramic substrates is described in more detail in U.S. Pat. No. 4,245,273. In the manufacture of multilayer ceramic substrates for integrated circuit semiconductor packages, it is imperative that shrinkage that occurs during the sintering operation be predictable and consistent, and also that the shrinkage be uniform throughout the multilayer ceramic substrate. Further, the stresses and material changes generated by the lamination process operate to cause variability during sintering.